Chapter 7 – Nervous system Appropriated from Mrs. Harborth Anatomy and physiology
Structural classification • Central nervous system (CNS) – consists of the brain and spinal cord. Command center • Peripheral nervous system (PNS) – part of body outside of CNS. • Spinal nerves – carry impulses to and from spinal cord • Cranial nerves – carry impulses to and from the brain
PNS • Sensory (afferent) – carries impulses FROM sensory organs TO the brain • Motor (efferent) – carries impulses FROM the brain TO effector organs, muscles, and glands. • Somatic nervous system – control skeletal muscles • Autonomic nervous system – regulates automatic events (smooth and cardiac muscle) • Sympathetic • Parasympathetic
Nervous tissue • Supporting cells of CNS “neuroglia” • Astrocytes • Microglia • Ependymal cells • Oligodendrocytes • Schwann cells • Satellite cells • Neurons • Cell body (Nissl substance & neurofibrils) • Processes (fibers) • Myelin sheath, Schwann cells (PNS), nodes of Ranvier • Dendrites (toward cell body) • Axons (away from cell body) • Axonal terminals, neurotransmitters, synaptic cleft, synapse
Astrocyte Ependymal cells Oligodendrocyte Microglial cell
Multiple sclerosis • Immune system gradually destroys myelin sheaths, converting to scleroses (hardened processes)
Vocab • Nuclei – clusters of cell bodies in the CNS • Ganglia – small collections of cell bodies found in the PNS. • Tracts – bundles of nerve fibers in the CNS • Nerves – bundles of nerve fibers in the PNS • Whitematter – myelinated fibers in the CNS • Graymatter – unmyelinated fibers and cell bodies in the CNS
Classification of neurons • Functional (direction of nerve impulse) • Sensory (afferent) neurons – neurons carrying impulses from sensory receptors to the CNS. Cell bodies are found in ganglion outside of CNS. • Cutaneous sense organs, proprioceptors • Motor (efferent) neurons – neurons carrying impulses to viscera and/or muscles and glands. Cell bodies are in the CNS. • Association neurons (interneurons) – connect motor and sensory neurons in neural pathways.
Classification of neurons • Structural • Multipolar neuron – several processes extending from cell body. • All motor and association neurons • Bipolar neuron – axon and dendrite extend from cell body • Rare. Found in some special sense organs. • Unipolar neuron– short, single process that usually divides into proximal and distal fibers. Axon conducts nerve impulses both toward AND away from the cell body. • Sensory neurons found in PNS ganglia
Neuron physiology • Nerve impulse – • Irritability • 1. Inactive neuron is polarized (more negative inside) • 2. Stimulus excites neuron, permeability increases • 3. Depolarization Action Potential • 4. Repolarization • Saltatory conduction – occurs on myelinated nerves FASTER • Cold and continuous pressure hinder conduction • Conductivity • When action potential reaches axonal endings, tiny vesicles fuse with axonal membranes, rupture, and releases neurotransmitters which start impulse in next neuron • Electrochemical event • animation
Neuron physiology • Reflex Arc • Autonomic reflexes • Regulate activity of smooth muscles, heart, and glands. • Ex: salivary reflex and pupillary reflex. • Somatic reflexes • Reflexes that stimulate skeletal muscles • Minimum of five elements: • Sensory receptor • Effector organ • Afferentneuron • Efferent neuron • Integration center
Central Nervous System • Embryonic development: • Neural tube develops • 4th week – anterior end of neural tube expands and brain formation begins. Central canal of neural tube becomes 4 ventricles
Functional Anatomy of the Brain • Cerebral Hemispheres • Diencephalon • Cerebellum • Brain Stem
Cerebral Hemispheres • Gyri: elevated ridges of tissue • Sulci: shallow grooves • Fissues: deeper grooves which separate large regions of the brain • Hemispheres separated by longitudinal fissure • Lobes are named for cranial bones that surrounding them
Cerebral Hemisphere • Parietal lobe: posterior to central sulcus • Somatic sensory area – localizes and interprets impulses from sensory receptors. Sensory pathways cross (left -> right, right -> left) • Occipital lobe: visual area • Temporal lobe: auditory and olfactory area • Frontal lobe: Higher intellectual reasoning in anterior part • Primary motor area - Axons of motor neurons form pyramidal (corticospinal) tract which descends to spinal cord. Pathways are crossed. • Broca’s area – speech. Only found in one hemisphere • Speech area – junction of temporal, parietal, and occipital lobes. Sound out words. One hemisphere. • Gray matter (cerebral cortex): cell bodies of neurons • White matter: fiber tracts carrying impulses to or from the cortex • Corpus callosum: large fiber tract that connects cerebral hemispheres. Allows halves to communicate with one another. • Basal nuclei: “islands” of gray matter buried in white matter. Modify instructions sent to muscles.
Diencephalon • Thalamus • Encloses third ventricle • Relay station for sensory impulses • Hypothalamus • Regulation of temperature, water balance and metabolism. Regulates pituitary • Limbic system – “emotional-visceral brain”. Thirst, appetite, sex, pain, pleasure centers. • Mammillary bodies – reflex involved in olfaction • Epithalamus • Pineal body • Choroid plexus of third ventricle
Diencephalon
Brain stem • Midbrain • Cerebral aqueduct – canal that connects third ventricle of diencphalon to fourth ventricle below • Cerebral peduncles – convey ascending and descending impulses • Corpora quadrigemina– reflex centers for vision and hearing • Pons • Mostly fiber tracts, but does have some nuclei involved in controlling breathing • Medulla oblongata • Merges into spinal cord. • Regulate vital visceral activities. Controls heart rate, blood pressure, breathing, swallowing, and vomiting • Reticular formation • Neurons involved in motor control of the visceral organs. RAS (reticular activating system) plays a role in consciousness and awake/sleep cycles. Damage to this area results in coma.
Brain Stem
Cerebellum • Two hemispheres • Timing for skeletal muscle activity and controls balance/equilibrium • Takes info from eyes, ears and muscles to coordinate movements in a smooth fashion
meninges • Three connective tissue membranes covering and protecting brain and spinal cord • Dura mater – outermost layer. Double-layered, forms periosteum and meningeal layer. • Arachnoid mater – span subarachnoid space to attach to innermost membrane • Subarachnoid space filled with cerebrospinal fluid. Arachnoid villi protrude through dura mater. Dural sinuses absorb fluid into blood through arachnoid villi. • Pia mater – delicate layer clings tightly to surface of brain and spinal cord. • Meningitis – inflammation of meninges
Cerebrospinal fluid (CSF) • similar to blood plasma. • continually formed from blood by choroid plexuses • continually moving. • Blood-brain barrier – any changes in fluid around brain could be detrimental. Barrier composed of the LEAST permeable capillaries in whole body. Water, glucose and essential amino acids can pass through.
Spinal Cord • 17 inches long, thickness of a thumb • Extends from foramen magnum to 1st or 2nd lumbar vertebrae, right below ribs • Caudaequina
Peripheral Nervous System • Endoneurium • Perineurium • Fascicles • Epineurium • Nerves are classified by which direction they transmit • Mixed nerves • Afferent (sensory) • Efferent (motor)
Cranial nerves • 12 pairs (table 7.1) • Serve head and neck (except vagus nerves – extend to thoracic and abdominal cavities) • I. Olfactory • II. Optic • III. Oculomotor • IV. Trochlear • V. Trigeminal • VI. Abducens • VII. Facial • VIII. Vestibulochochlear • IX. Glossopharyngeal • X. Vagus • XI. Accessory • XII. Hypoglossal
Spinal Nerves/ Nerve Plexuses • 31 pairs of spinal nerves formed by combo of ventral and dorsal roots of spinal cord. Named for region of spinal cord from which they arise. • Each spinal nerve divides into dorsal and ventral rami • Dorsal rami serve skin and muscles of posterior body trunk • Ventral rami of T1 – T12 form intercostal nerves • Ventral rami of other spinal nerves form 4 plexuses (Table 7.2)
Autonomic nervous system • Motor subdivisions that control automatic activities (cardiac and smooth muscle, and glands) • Sympathetic and parasympathetic • Sympathetic – mobilizes body during extreme situation (fear, exercise, rage, etc.) • Parasympathetic – allows us to “unwind” and conserve energy
